1. Field of the Invention
The present invention relates to an improved magnetic recording medium, more particularly, to a magnetic recording medium (e.g. magnetic tape and sheet) which has improved surface properties and wear resistance, as well as increased dispersibility of magnetic particles with the magnetic layer.
2. Description of the Prior Art
Conventional magnetic recording media such as audio tape and video tape are manufactured by applying a magnetic mix (paint) onto a non-magnetic film substrate such as polyethylene terephthalate, triacetyl cellulose, polycarbonate, polypropylene or polyvinyl chloride. The magnetic mix is a dispersion of magnetic particles in a binder selected from among vinyl chloride/vinyl acetate copolymers, vinyl chloride/vinylidene chloride copolymer, vinylidene chloride/acrylonitrile copolymers, butyl acrylate/acrylonitrile copolymers, cellulose resins, epoxy resins and polyurethane resins.
The conventional binders, whether they are used alone or in combination, have proved unsatisfactory in providing magnetic recording media with improved surface properties and wear resistance, as well as increased dispersibility of ferromagnetic particles. With video tape that is often required to record short-wavelength signals, an insufficient dispersion of magnetic particles in the magnetic layer reduces the S/N ratio and reproduction output power. Furthermore, the conventional magnetic recording m media keep running in intimate contact with the magnetic head in recording and reproducing modes, and as a result of repeated use, the magnetic layer wears to increase the chance of the dislodging of the magnetic particles. A plausible reason for this phenomenon is that the particles of a ferromagnetic material such as gamma-Fe.sub.2 O.sub.3 have a hydrophilic surface which does not have high affinity for the binder, so they are not dispersed completely uniformly in the binder. Japanese Patent Application (OPI) No. 64330/82 (the symbol OPI as used herein means an unexamined published Japanese patent application) proposes increasing the affinity of the ferromagnetic powder for the binder by using a titanate coupling agent such as isopropyl tris(dioctylpyrophosphate)titanate, isopropyl tridecylbenzenesulfonyltitanate, bis(dioctylpyrophosphate)ethylene titanate, or di(dioctylpyrophosphate)oxyacetate titanate. However, the improvement achieved by this technique is not completely satisfactory. It is well known to use a vinyl chloride/vinyl acetate copolymer and a polyurethane resin as binders for video tape, mm and the technique of coating on a substrate with a magnetic mix having ferromagnetic particles dispersed in a mixture of these binders and the titanate coupling agent is shown in Japanese Patent Application (OPI) Nos. 111829/82, 64330/82 and 58135/81.
This combination of the binders and thitanate coupling agent has achieved some improvement, but it is not completely satisfactory.
Japanese Patent Application (OPI) No. 3227/82 shows the production of a stable magnetic layer by mixing a binder with tetra(2,2-diallyloxymethyl-1-butyl)bis(ditridecylphosphite)titanate ("KR-55", product by Kenrich Photochemicals, Inc.) which has 6 ligands, as well as an ethylenically double bond and allyl group, dispersing ferromagnetic particles in the mixture, coating the resulting magnetic mix onto a non-magnetic substrate, and exposing the web to the radiation. However, the magnetic layer formed by coating the magnetic mix in the presence of a coating solvent and drying the web with hot air cannot be stored for an extended period without causing deterioration.
It has been known that a greager wear resistance can be obtained by using nitrocellulose as a binder. However, one defect with the nitrocellulose is its poor adhesion to the non-magnetic substrate. In order to provide an improved adhesion, the use of a known dispersant such as lecithin or sulfate esters has been proposed, but with unsatisfactory results. Furthermore, nitrocellulose turns red when it is dissolved in a solvent together with a cationic dispersant.